Innovative solder ball pad structure to ease design rule, methods of fabricating same and substrates, electronic device assemblies and systems employing same

ABSTRACT

A solder ball pad for mounting and connecting of electronic devices and, more particularly, apparatus and methods providing an improved solder ball pad structure on a substrate, such as a printed circuit board (“PCB”) or a semiconductor die, while enabling better use of the spaces between adjacent solder ball pads and at the same time providing increased surface area for bonding to a solder ball. More particularly, the inventive solder ball pad structure comprises a terminal pad exposed through an aperture in an insulative mask having a bond pad layer comprising at least another metal layer formed over, at most, a portion of the exposed portion of the terminal pad. Methods of manufacture and substrates incorporating same are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.10/230,962, filed Aug. 29, 2002, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the mounting andconnecting of electronic devices and, more particularly, to apparatusand methods providing an improved solder ball pad structure on asubstrate such as a printed circuit board (“PCB”) or a semiconductordie.

[0004] 2. State of the Art

[0005] An increasing demand for electronic equipment that is smaller,lighter, and more compact has resulted in a concomitant demand forsemiconductor packages that have smaller outlines and mounting areas or“footprints.”

[0006] One response to this demand has been the development of theso-called “flip-chip” method of attachment and connection ofsemiconductor chips to substrates. Sometimes referred to as the“Controlled Collapse Chip Connection,” or “C4,” method, the techniqueinvolves forming balls of a conductive metal, e.g., solder or gold, oninput/output connection pads on the active surface of the chip, theninverting, or “flipping” the chip upside down, and “reflowing” theconductive balls, i.e., heating them to the melting point, to fuse themto corresponding connection pads on a substrate.

[0007] Another response has been the development of a so-called ballgrid array (“BGA”) semiconductor package that “surface mounts” andelectrically connects to an associated carrier substrate, e.g., aprinted circuit board (“PCB”), with a plurality of solder balls in amethod sometimes referred to as the “C5” method that is analogous to theflip-chip method described above for mounting and connecting dies.

[0008] In both the C4 die and C5 package mounting and connectionmethods, a plurality of solder balls is attached to respective solderball mounting lands, or pads, defined on a surface of the die orinterposer substrate. The solder ball mounting pad may be defined by anopening in an insulative layer or mask called a “passivation layer” inthe case of a semiconductor die, or a “solder mask” in the case of aninterposer substrate of a BGA package, as described below. Theinterposer substrate in a BGA package may comprise a rigid or flexiblesheet material.

[0009] In a solder-mask-defined (“SMD”) solder ball pad, an apertureformed in the mask over a terminal pad defines the solder ball padmounting area. Typically, the terminal pad comprises a layer of metal,e.g., copper, aluminum, gold, silver, nickel, tin, platinum, or amultilayer combination of the aforementioned that has been laminatedand/or plated on a surface of the substrate sheet and then patternedusing known photolithography techniques. Further, one or more circuittraces may be formed simultaneously with the terminal pads using thesame processes. In addition, a plated through-hole, called a “via,” mayalso be formed and may connect the pad layer with the opposite surfaceof the substrate sheet.

[0010] A solder mask is then formed over the metal terminal pad and maycomprise an acrylic or a polyimide plastic or, alternatively, an epoxyresin that is silk screened, spin-coated or applied as a preformed filmon the substrate sheet. An aperture is formed in the solder mask toexpose a portion of the terminal pad, but not any portion of thesurrounding substrate surface. A solder ball may be attached to orformed on the terminal pad area thus exposed; however, the solder maskprevents the solder of the solder ball from attaching to any portion ofthe terminal pad other than the mounting area that is exposed throughthe aperture. Thus, the exposed area is referred to as an SMD-type ofsolder ball mounting pad.

[0011] Comparatively, a nonsolder-mask-defined (“NSMD”) solder ballmounting pad may be formed in a similar manner, the exception being thesize of the aperture in the solder mask. In particular, typically, theNSMD pad exposes the entire terminal pad, at least a portion of thesurface of the substrate sheet and, optionally, a portion of an adjacentcircuit trace, such that the molten solder of the solder ball can attachto the entire surface and peripheral vertical side surface of theterminal pad thus exposed. Typically, a circular-shaped terminal pad anda portion of a circuit trace are exposed in an NSMD solder ball mountingpad arrangement. The connection area of both the SMD-type and NSMD-typesolder ball mounting pads may be coated with a nickel layer and then agold layer to enhance wettability of solder thereon.

[0012] Each of the conventional SMD and the NSMD solder ball mountingpads have some advantages as well as disadvantages associated with it.

[0013] Turning to the SMD solder ball pad, it provides relatively good“end-of-line” (i.e., at the end of the semiconductor package fabricationline) ball shear resistance because the solder mask overlaps theperipheral edge of the terminal pad proximate to the exposed areadefining the solder ball mounting pad and, therefore, resists ripping ofthe terminal pad from the substrate when mechanical forces act on thesolder ball attached thereto. In contrast, the NSMD solder ball pad hasa relatively lower end-of-life shear resistance because the solder maskdoes not cover the peripheral edge of the NSMD terminal pad.

[0014] The SMD solder ball pad also affords relatively better control ofthe lateral (x-y) position of the solder ball on the surface of thesubstrate than does an NSMD solder ball pad. This is because the lateralposition of the solder ball on the substrate may be affected by twofactors: 1) the position on the substrate of the centroid of theaperture in the solder mask, if the vertical wall of the apertureinteracts (e.g., touches, or electrostatically interacts) with thesolder ball, and 2) the position of the centroid of the area of themetal pad layer that is exposed by the opening in the mask, i.e., thearea wetted by the molten solder of the solder ball when the latter isattached to the solder ball pad. In both instances, the center ofgravity of the solder ball tends to align itself over each of the tworespective centroids if both factors apply. As a result, when thecentroid of the aperture does not coincide with the centroid of theexposed area of the mounting pad and the vertical wall of the apertureinteracts with (e.g. touches) the solder ball, the center of gravity ofthe solder ball may be positioned approximately half way along a lineextending between the two centroids. Since in an SMD solder ball pad theaperture in the solder mask exposes only pad layer metal, the centroidof the aperture and exposed metal pad layer coincide. Thus, so long asthe aperture in the solder mask is located within the periphery of themetal pad layer, the lateral tolerances of the SMD solder ball willdepend substantially on the lateral positional tolerances on thecentroid of the aperture.

[0015] However, the shape of the NSMD solder ball pad exposed by theaperture in the solder mask includes a terminal pad portion as well as aportion of the circuit trace. Further, the vertical wall of the aperturemay not touch the solder ball. Consequently, the centroid of the NSMDsolder ball pad, i.e., of the exposed area of metal, is shifted slightlytoward the circuit trace and away from the centroid of the opening,which is typically centered on the terminal pad portion. Hence, thecenter of gravity of the solder ball will be positioned according to therespective centroids of the NSMD solder ball pad and the circuit trace.Thus, the lateral tolerances on the solder ball on an NSMD solder ballpad may depend not only on the lateral tolerances of the centroid of theaperture, but also the lateral tolerances of the centroid of the exposedmetal of the metal pad layer as well. Moreover, even without thepresence of a circuit trace, misalignment of the solder ball can stilloccur in an NSMD pad if the centroid of the exposed pad is notsufficiently aligned with the centroid of the aperture, and thus avertical sidewall of the aperture interacts with the solder ball.

[0016] While the lateral misalignment of a solder ball relative to anopening resulting from this “shift” is relatively small, it should beunderstood that a C4-mounted die or a C5-mounted semiconductor packagecan typically have a large number, e.g., up to nine hundred, of suchsolder balls on its mounting surface, and that accordingly, these slightmisalignments in the array of balls can be additive, such that in somecases, the die or package cannot be successfully mounted to anassociated mounting surface.

[0017] As a further comparison between SMD and NSMD solder ball pads,the solder ball attached to an NSMD solder ball pad attaches to thevertical side surface of the exposed metal of the terminal pad includingthe circuit trace(s), if any. It is postulated that this side surfaceattachment and resulting arcuate attachment structure helps todistribute stresses resulting from thermal aging so that the stresses donot concentrate at the interface between the NSMD solder ball pad andthe solder ball. Thus, the NSMD may provide an improved resistance tothermal stresses over the SMD solder ball pad, the solder ball/padinterface of which consists of a simple planar interface between theexposed portion of the terminal pad and the solder ball.

[0018] U.S. Pat. No. 6,201,305 to Darveaux et al. as well as U.S. Pat.No. 5,872,399 to Lee each describes a solder ball pad structure. Morespecifically, the Darveaux reference describes an NSMD-type solder ballpad structure wherein a layer of metal on the substrate is formed into aterminal pad, the pad having at least two spokes radiating outwardlyfrom it. The pad structure with spokes is exposed by way of an apertureformed through the solder mask such that the terminal pad and an innerportion of each of the spokes is exposed therethrough, and an outerportion of each of the spokes is covered by the mask. The Lee referencedescribes a solder ball pad structure having a terminal etching hole aswell as a plurality of etching holes at the outer portion of the solderball pad structure for increasing the contact area for a solder ball.

[0019] Another area of interest is the design flexibility in the numberof circuit traces that may be operably positioned to run between twoadjacent solder ball pads with adequate spacing between the traces andbetween the traces and the solder ball pads. More specifically, theaforementioned tolerance considerations, as well as the differences inthe formation of SMD and NSMD solder ball pads, must be factored indetermining the spacing between circuit traces and solder ball pads. Ofcourse, dimensional tolerances as well as parameters required to achievea robust design limit the ability to position additional circuit tracesbetween solder ball pads for a given solder ball pad design pitch.

[0020] In view of the foregoing, a method for fabricating solder ballmounting pads on a substrate and resulting solder ball mounting padswhich improve on both types of conventional solder ball pads andeliminates some of their respective disadvantages would be desirable.

BRIEF SUMMARY OF THE INVENTION

[0021] The present invention comprises an apparatus and method providingan inventive solder ball pad structure and substrates, electronic deviceassemblies and systems employing same. The solder ball pad structure ofthe present invention includes a metal terminal pad that is partiallyexposed by an aperture in a solder mask layer which does not expose anyportion of a surrounding substrate surface. In addition, an optionalmetallic or conductive polymer interface layer may be formed onto atleast a portion of the exposed area of the metal terminal pad and ontoat least a portion of the vertical sidewall of the solder mask definingthe aperture as well as extending onto the surrounding top horizontalsurface of the solder mask. Alternatively, an optional nonconductivepolymer interface layer may be formed onto the surrounding top surfaceof the solder mask. A copper layer comprising an electroless copper seedlayer as well as an optional electroplated copper layer may be formedover the solder mask, into the aperture and over the exposed portion ofthe terminal pad. Nickel and gold layers may be applied byelectroplating to the copper layer to enhance the wettability to solderof the resulting pad surface. The solder ball pad structure of thepresent invention may be termed a ball pad on solder mask or “BPS.”

[0022] The solder ball pad structure of the present invention provides avariety of advantages. First, the lateral positional tolerance of anattached solder ball is largely determined by the tolerances associatedwith the formation of the solder mask, similar to the SMD solder ballpad. Additionally, the optional interface layer and subsequent metallayers which may be attached to the metal terminal pad enable the solderball to attach to the vertical side surface of the aperture in theresulting structure, which configuration may provide enhanced thermalstress distribution in the solder ball connection. Also, the solder ballpad of the present invention also provides increased surface area forsolder ball attachment, as well as an indented surface for attachmentwhich may further strengthen the bond between a solder ball and theinventive solder ball pad structure.

[0023] As another advantage, the solder ball pad structure of thepresent invention provides additional, usable lateral space on thesubstrate between adjacent solder ball pads for circuit traces andcircuit trace spacing. Because the layered structure above the metalterminal pad on the substrate increases the area for attachment for thesolder ball, in excess of the metal pad layer area that is exposed viathe solder mask aperture, the size of the metal terminal pad may bereduced. This enables additional circuit traces or spacing to beemployed between adjacent solder ball pads, as desired.

[0024] It is also contemplated that the structure of the presentinvention has utility with discrete conductive elements other thansolder, such as conductive or conductor-filled epoxy. Accordingly, theterm “solder ball pad” is exemplary and not limiting of the scope of thepresent invention.

[0025] Other features and advantages of the present invention willbecome apparent to those of ordinary skill in the art throughconsideration of the ensuing description, the accompanying drawings, andthe appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0026] In the drawings, which illustrate what is currently considered tobe the best mode for carrying out the invention:

[0027]FIG. 1A is a top view of a conventional SMD solder ball pad;

[0028]FIG. 1B is side cross-sectional view of the SMD solder ball padshown in FIG. 1A;

[0029]FIG. 2A is a top view of a conventional NSMD solder ball pad;

[0030]FIG. 2B is side cross-sectional view of the NSMD solder ball padshown in FIG. 2A;

[0031]FIG. 3A is a top view of an embodiment of the solder ball padstructure of the present invention;

[0032]FIG. 3B is a side cross-sectional view of an embodiment of thesolder ball pad structure shown in FIG. 3A;

[0033]FIG. 4A is a top view of an SMD solder ball pad configurationwherein two circuit traces extend between two SMD solder ball mountingpads;

[0034]FIG. 4B is a side cross-sectional view of the SMD solder ball padconfiguration shown in FIG. 4A;

[0035]FIG. 5A a top view of a solder ball pad configuration of thepresent invention wherein three circuit traces extend between two solderball mounting pads;

[0036]FIG. 5B is a side cross-sectional view of the solder ball padconfiguration of the present invention shown in FIG. 5A;

[0037]FIG. 6A is a top view of an NSMD solder ball pad configurationwherein two circuit traces extend between two NSMD solder ball mountingpads;

[0038]FIG. 6B is a side cross-sectional view of the NSMD solder ball padconfiguration shown in FIG. 6A;

[0039]FIGS. 7A through 7L show top views and associated sidecross-sectional views of different embodiments of the present invention;

[0040]FIGS. 8A and 8B depict an exemplary process flow for forming thesolder ball pad structure of the present invention using a conductivepolymer interface layer;

[0041]FIG. 9 depicts placement of a nonconductive polymer interfacelayer in accordance with the present invention;

[0042]FIGS. 10A through 10D depict a first exemplary process flow forforming the solder ball pad structure of the present invention using anelectrolessly plated interface layer; and

[0043]FIGS. 11A through 11D depict a second exemplary process flow forforming the solder ball pad structure of the present invention using anelectrolessly plated interface layer.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Referring again to conventional practices to provide a moredetailed basis for comparison with the present invention and not in anyway in limitation of the scope thereof, FIG. 1A is a top view of aportion of a conventional SMD substrate 10 having a solder-mask-defined(“SMD”) solder ball mounting pad 28 formed thereon. FIG. 1B is across-sectional view looking into the SMD substrate 10 and mounting pad28 along the lines IB-IB in FIG. 1A. The SMD substrate 10 may comprise asheet 12 of an insulative material, such as bismaleimide triazine,flexible polyimide film or tape, fiberglass, polyimide tape, ceramic, orsilicon, or, alternatively, it may comprise a semiconductor chip or die.The SMD substrate 10 typically comprises a layer of metal, e.g., copper,aluminum, gold, silver, nickel, tin, platinum, or a combination of theforegoing that has been laminated and/or plated on a surface of thesubstrate sheet 12, then patterned using known photolithographytechniques into a terminal pad 14, which may include one or more circuittraces 16 (shown by dotted lines) extending from it. In addition to thecircuit traces 16, a plated through-hole, called a “via” (not shown),may connect the terminal pad 14 with the opposite surface of thesubstrate sheet 12 as known in the art.

[0045] An insulative layer in the form of solder mask 20 is formed overthe metal layer, including the terminal pad 14. The solder mask 20 maycomprise an acrylic or a polyimide plastic or, alternatively, an epoxyresin that is silk screened or spin-coated on the sheet 12. A dry filmsolder mask may also be employed. An aperture 19 is formed in the soldermask 20 to expose a mounting pad portion 28 of the terminal pad 14, anda solder ball 24 (shown dotted in FIG. 1A) is attached to or formed onthe mounting pad 28 thus exposed. Since the solder mask 20 prevents thesolder of the solder ball 24 from attaching to any portion of theterminal pad 14 other than the mounting pad 28 that is exposed throughthe aperture 19, the mounting pad 28 is referred to as asolder-mask-defined or SMD-type of solder ball mounting pad, asdescribed above.

[0046] In further illustration of conventional practices for purposes ofcomparison with the present invention and not in any way in limitationof the scope thereof, a conventional NSMD substrate 11 is illustrated inthe top view of FIG. 2A, wherein features similar to those in the SMDsubstrate 10 of FIG. 1A are numbered similarly. FIG. 2B is across-sectional view looking into the NSMD substrate 11 and mounting pad28′ along the section lines IIB-IIB in FIG. 2A.

[0047] As may be seen from a comparison of the two sets of figures, therespective mounting pads 28 and 28′ are very similar, the exceptionbeing the relative sizes of the apertures 19 and 19′ in the solder mask20. In particular, in the NSMD mounting pad 28′ of FIGS. 2A and 2B, theaperture 19′ exposes the entire terminal pad 14, along with a portion ofthe surface of the substrate sheet 12 and a portion of the optional,adjacent circuit trace 16, such that the molten solder of the solderball 24 can wet and attach to not only the entire upper surface of theterminal pad 14, but also to the vertical peripheral side surface 26 ofthe terminal pad 14 and the optional circuit trace 16. Along thevertical side surface 26 of the terminal pad 14, the solder ball 24attaches and forms a curved attachment surface 29 with the verticalperipheral side surface 26 of the terminal pad 14.

[0048] It is conventional in the industry to plate solder ball mountingpads 28 and 28′ with a layer of nickel, followed by a layer of gold,shown in combination in FIGS. 11B and 2B as solderability enhancementlayer 18, to improve the solderability of the pads. Alternatively,terminal pads 14 may be supplied with nickel/gold, tin/lead, or silvercoatings, or may be treated to prevent oxidation of the metal surface ofthe terminal pad 14.

[0049]FIGS. 3A and 3B show a top and side cross-sectional view of theBPS substrate 40 with solder ball mounting pad 36 according to thepresent invention. Solder mask 20 exposes an area of the terminal pad 41on insulative material sheet 12 by way of aperture 23. Interface layer38 is formed onto the exposed surface area of the terminal pad as wellas extending onto the vertical sidewall of the aperture 23 and onto thetop horizontal surface of the solder mask 20. Interface layer 38 may beused to enhance the adhesion of the subsequent copper layer to thesolder mask 20 surface, and may comprise an epoxy, such as HYSOL® E01073or E01075, from Henkel Loctite Corporation, Connecticut. Interface layer38 may optionally comprise a metal layer formed by using an electrolessplating solution or a conductive polymer, as described in more detailbelow. Copper layer 48 is formed over the terminal pad 41 as well asinterface layer 38, if present, thus extending along the horizontalportion of the terminal pad 41 and onto the sidewall of the solder mask20 defining aperture 23, and also onto the horizontal top surface of thesolder mask 20. Copper layer 48 may comprise an electroless copper seedlayer (which may be the interface layer 38) followed by an electroplatedcopper layer or may be otherwise formed as known in the art. Further,nickel and gold layers, collectively shown as solderability enhancementlayer 18 for clarity, may be applied to the copper layer 48 to enhancethe wettability to solder of the resulting mounting pad surface. Nickelis used to prevent diffusion of copper to the solder ball pad surfaceand gold is used for solder wettability. Thus, optional interface layer38, copper layer 48 and solderability enhancement layer 18 togethercomprise a solder ball pad layer 60.

[0050] Because the interface layer 38 as well as the copper layer 48 andsolderability enhancement layer 18, due to their extension up thesidewall of solder mask 20 defining aperture 23 and over onto the outersurface of solder mask 20, may provide a larger surface area than thearea that would be exposed by aperture 19 in a typical SMD-type solderball pad, the size of terminal pad 41 of a BPS solder ball pad structuremay be accordingly reduced. Stated another way, to achieve a finalbonding area that is equal to a given SMD mounting pad area, theterminal pad 41 formed from the metal layer deposited on the surface ofthe substrate sheet may be smaller than the terminal pad 14 of an SMD orNSMD configuration. Reducing the size of terminal pad 41 may allow formore lateral space between adjacent terminal pads to become available onthe surface of sheet 12. By way of example only, solder ball pad layer60 may exhibit a diameter of about 0.33 millimeters or larger and atotal surface area of about 0.05 square millimeters or greater.

[0051] Moreover, the combination of interface layer 38, copper layer 48,and solderability enhancement layer 18 may comprise a multitude ofconfigurations. For instance, each layer may be formed in selected areasto improve solder ball bonding characteristics. More particularly, theinterface layer 38 may only be deposited over the solder mask 20, orover selected portions of the solder mask 20 to anchor the subsequentlayers thereto. Similarly, the copper layer 48 and solderabilityenhancement layer 18 may be configured in different arrangements aswell. Furthermore, aforementioned layers comprising the BPS solder ballpad structure may be disparate areas that are not contiguous orcontinuous. Thus, it may be desired to form separate copper regions thatform the copper layer 48. Likewise, separate solderability enhancementregions may, in combination, form the solderability enhancement layer18. Also, each layer is not required to be the same size as otherlayers. For instance, the solderability enhancement layer 18 may extendonto the vertical side of the copper layer 48, or may extend laterallyalong the substrate surface beyond either the copper layer 48 and/orinterface layer 38.

[0052] Suitable and exemplary rigid insulative sheet materials 12 for aBPS substrate include BT832, MGC, MCL679, FR-4, FR-5 materials fromHitachi Co., Japan. Suitable and exemplary flexible insulative sheetmaterials 12 for a BPS substrate include polyimide layers or fibers suchas UPILEX™ from Ube Industries Ltd., Japan, ESPANEX™ from Nippon SteelChemical Co. Ltd., Japan, and KAPTON™ and MICROLUX™ commerciallyavailable from E.I. Dupont de Nemours Company, as well asPolytetrafluoroethylene (PTFE), and a liquid crystal polymer. It shouldalso be noted that the term “sheet” as used herein encompasses not onlya self-supporting structure but a layer of material supported on anotherstructure.

[0053] AUS5, AUS308, AUS303, or AUS7 from Taiyo, Japan, and DSR2200 fromTamura, Japan, are examples of commercially available materials suitablefor use in forming solder masks 20 for a rigid BPS substrate. AUS11,AUS21 and PSR8000FLX from Taiyo, Japan and CFP1122 and CFP1123 fromSumilite, Japan, are exemplary materials suitable for use with flexibleBPS substrates.

[0054]FIG. 4A shows a conventional SMD substrate 10 configuration havingtwo solder ball mounting pads 28, formed by apertures 19 defined bysidewalls 21 (FIG. 4B) of solder mask 20 that expose mounting pads ofthe terminal pads 14 formed on the substrate sheet 12, respectively. Thedistance between terminal pads 14 as well as tolerances in positioningthe terminal pads 14 may substantially influence the amount of space inwhich to position conductive traces 30 and 32 extending between solderball mounting pads 28. The spacing between traces 30 and 32 isdetermined from a number of variables. The distance between the centersof the terminal pads 14, termed “solder ball pad pitch,” the terminalpad diameter, the conductive trace thickness t, the number of conductivetraces, the lateral tolerance in forming conductive traces 30 and 32 andterminal pads 14, as well as the solder ball pad design all mayinfluence the spacing d that may be afforded for placement of conductivetraces 30 and 32 in relation to the terminal pads 14 on the substratesheet 12. In addition, it is common for the trace thickness t to beequal to the spacing between the traces.

[0055]FIG. 4B shows a side cross-sectional view of the SMD substrate 10shown in FIG. 4A, but also including an example of a solder ball 24 (notshown in FIG. 4A) attached to the left-hand mounting pad 28. Thedistance d between a terminal pad 14 and conductive trace 30, conductivetrace 30 and conductive trace 32, as well as conductive trace 32 andanother terminal pad 14 is shown. For ease of illustration, trace orline widths and space widths are taken to be substantially the same.Distance d, the spacing between a trace and another trace or a trace anda terminal pad for SMD-type solder ball pad configurations, may bedetermined by the following design rule: $\begin{matrix}{d_{SMD} = \frac{S_{pitch} - P_{dia} - {2{Tol}}}{N}} & {{Equation}\quad 1}\end{matrix}$

[0056] Where:

[0057] d_(SMD) is the spacing between a trace and another trace or atrace and a terminal pad.

[0058] S_(pitch) is the solder ball pad pitch, or distance between thecenters of the two pads.

[0059] P_(dia) is the terminal pad diameter.

[0060] Tol is the soldermask positional tolerance.

[0061] N is the number of spaces and traces required.

[0062] Applying Equation 1 to FIGS. 4A and 4B, with assumed dimensionsas follows,

[0063] S_(pitch)=0.650 mm

[0064] P_(dia)=0.300 mm

[0065] Tol=0.050 mm

[0066] N=5 (As can be seen in FIG. 4B, the number of spaces “d” is 5 fortwo circuit traces and three intervening spaces between pads, assumingequal spacing and trace widths)$d_{SMD} = \frac{{{.650}\quad {mm}} - {{.300}\quad {mm}} - {{2 \cdot {.050}}\quad {mm}}}{5}$

 d _(SMD)=0.050 mm (two circuit traces)

[0067] Comparatively, FIGS. 5A and 5B show a top and sidecross-sectional view, respectively, of an embodiment of the BPSsubstrate 40 of the present invention. Although none of the drawings aredrawn to scale and are for illustration purposes only, the sizes of theBPS mounting pads 36, as defined by apertures 23, respectively, areshown as substantially equal to the mounting pad 28 size as shown FIG.4A. However, the terminal pads 41 of the BPS substrate 40 are smallerthan the terminal pads 14 of the SMD substrate 10 shown in FIGS. 4A and4B. Since the BPS mounting pads 36 are formed onto the top surface ofthe solder mask 20, their outer extents do not influence the placementof conductive traces 30, 32 and 34. Instead, the smaller terminal pads41 may allow for additional spacing or additional conductive traces tobe placed between BPS terminal pads 41. Further, since the copper layer48 extends over the solder mask 20, it may be, for example, a diameteror width of at least 0.3 mm and preferably 0.35 mm to ensure good solderjoint reliability using BPS mounting pads 36 without any reduction incircuit trace or spacing width.

[0068] Determining circuit trace spacing of the BPS solder ball padconfiguration of the present invention may be accomplished by usingEquation 1, used for SMD solder ball pad configurations; however, theterminal pad size may be reduced.

[0069] For instance, applying the design rule of Equation 1 to theembodiment of the present invention shown in FIGS. 5A and 5B, withassumed dimensions as follows,

[0070] S_(pitch)=0.650 mm

[0071] P_(dia)=0.150 mm

[0072] Tol=0.050 mm

[0073] N=7 (As can be seen in FIG. 5B, the number of spaces “d” is sevenfor three circuit traces and four intervening spaces between pads)$d_{BPS} = \frac{{{.650}\quad {mm}} - {{.300}\quad {mm}} - {{2 \cdot {.050}}\quad {mm}}}{7}$

 d _(BPS)=0.057 mm

[0074] Accordingly, the present invention may enable an increased numberof traces to be placed between two solder ball pads of the presentinvention since the spacing size d may remain substantially identical tothe spacing required for conventional bond pads having a smaller numberof traces therebetween. Alternatively, additional space may be used toprovide additional lateral clearance between the same number of traces;thus increased yield may result.

[0075] For example, employing the BPS solder ball configuration of thepresent invention wherein two traces extend between two solder ballpads, the spacing may be determined as follows:

[0076] S_(pitch)=0.650 mm

[0077] P_(dia)=0.150 mm

[0078] Tol=0.050 mm

[0079] N=5 (for two circuit traces and three intervening spaces)$d_{BPS} = \frac{{{.650}\quad {mm}} - {{.150}\quad {mm}} - {{2 \cdot {.050}}\quad {mm}}}{5}$

 d _(BPS)=0.80 mm

[0080] Thus, the present invention may enable more traces to be formedbetween solder ball pads of the present invention and/or alternatively,increased spacing between a trace and another trace or a trace and aterminal pad, as well as in circuit trace width, in relation to theconventional SMD pad configuration.

[0081]FIGS. 6A and 6B show a conventional NSMD substrate 11 wherein twotraces 30 and 32 extend between the terminal pads 14. FIG. 6A showsapertures 19′ exposing the entire terminal pads 14, terminal pads 14forming mounting pads 28′.

[0082]FIG. 6B shows a side cross-sectional view of the solder ball padconfiguration shown in FIG. 6A, but also including an example of asolder ball 24 (not shown in FIG. 6A) attached to left-hand mounting pad28′. The distance d between a terminal pad 14 and conductive trace 30,conductive trace 30 and conductive trace 32, as well as conductive trace32 and a terminal pad 14 is shown. Distance d, the spacing between atrace and another trace or a trace and a terminal pad, is also commonlyused as the trace width as well as the spacing distance between thevertical sidewall 21 of aperture 19′ and a terminal pad 14. In addition,in an NSMD-type substrate, it is common for the design rule to specify aclearance distance between a vertical sidewall of an aperture, forinstance vertical sidewall 21 of aperture 19′, and the nearest sidewallof a trace, for instance, conductive trace 30.

[0083] Distance d, for the NSMD solder ball pad configuration shown inFIGS. 6A and 6B, may be determined by the following design rule:$\begin{matrix}{d_{NSMD} = \frac{S_{pitch} - {Aperture}_{dia} - {2{Tol}} - {2C}}{N}} & {{Equation}\quad 2}\end{matrix}$

[0084] Where:

[0085] d_(NSMD) is the spacing between a trace and another trace or atrace and a terminal pad.

[0086] S_(pitch) is the solder ball pad pitch, or distance between thecenters of the two pads.

[0087] Aperture_(dia) is the aperture diameter.

[0088] Tol is the solder mask positional tolerance.

[0089] C is the trace clearance, to ensure that the solder mask coversthe trace.

[0090] N is the number of traces required and intervening spaces betweenthe total number of traces.

[0091] For example, applying Equation 2 to two traces lying between twoNSMD solder ball pads, as shown in FIGS. 6A and 6B, the spacing may bedetermined as follows:

[0092] S_(pitch)=0.650 mm

[0093] Aperture_(dia)=0.300 mm

[0094] Tol=0.050 mm

[0095] C=0.030 mm

[0096] N=3 (for two traces and one intervening space between traces)$d_{NSMD} = \frac{{{.650}\quad {mm}} - {{.300}\quad {mm}} - {{2 \cdot {.050}}\quad {mm}} - {{2 \cdot {.030}}\quad {mm}}}{3}$

 d _(NSMD)=0.063 mm

[0097] Referring back to FIGS. 5A and 5B, the BPS substrate 40 of thepresent invention may offer increased spacing distance between elementsformed on the surface of the substrate when compared to the SMDsubstrate 10 or the NSMD substrate 11 for an equal number of circuittraces. Also, since the BPS mounting pad 36 is positioned in part abovethe solder mask 20, the BPS mounting pad size may offer a surface areaequal to or greater than the SMD mounting pad 28 and/or the NSMDmounting pad 28′ sizes.

[0098] In addition, the BPS substrate 40 of the present invention mayoffer increased flexibility in design and improved bondingconfigurations. For instance, a BPS solder ball pad layer may beconfigured to further increase the surface area for attaching a solderball thereto. More particularly, a BPS solder ball pad layer may beconfigured with scallops, radially extending fingers, apertures, orotherwise geometrically configured to increase the surface area orimprove the bonding characteristics of a solder ball connection thereto.Further, a BPS solder ball pad layer may be configured to expose aportion of the terminal pad for connection to a solder ball incombination with the solder ball connection surface of the BPS solderball pad layer. Additionally, as patterning of metal, and specificallycopper, to define a solder ball pad layer is more accurate than soldermask patterning, a BPS solder ball pad layer offers superior positionaland size tolerances as compared to an SMD solder ball mounting pad.

[0099]FIGS. 7A through 7L show different embodiments 99, 101, 103, 105,107 and 109 for a BPS solder ball pad layer 60 of the present inventionwherein the solder ball pad layer 60 comprises optional interface layer38, copper layer 48 as well as solderability enhancement layer 18, butis shown as a single layer 60 in FIGS. 7A through 7L for clarity. FIGS.7A and 7B show embodiment 99 including a solder ball pad layer 60configured generally in a circular area wherein the solder ball padlayer 60 includes an aperture 82 therethrough, exposing area 90 ofterminal pad 41. Area 90 may include a solderability enhancement layer18, although this is not shown for clarity. Thus, a solder ball attachedto the BPS solder ball pad embodiment shown in FIGS. 7A and 7B may beaffixed to the mounting pad 36, mounting pad 36 comprising area 80 ofsolder ball pad layer 60 including side surface 71 and exposed area 90of terminal pad 41.

[0100]FIGS. 7C and 7D show embodiment 101 including a solder ball padlayer 60 wherein solder ball pad layer 60 includes a terminal aperture82 exposing area 90 of terminal pad 41. In addition, scallops 73 areformed circumferentially about area 80 of solder ball pad layer 60.Scallops, fingers, spokes, or other laterally extending shapes may beadvantageous to increase the surface area of attachment for a solderball, as well as provide additional vertical surfaces for attachment ofa solder ball thereto.

[0101] For example, FIGS. 7E and 7F show embodiment 103 including asolder ball pad layer 60 having a terminal aperture 82 exposing area 90of terminal pad 41 and extending elements 81 configured as radiallyextending elements generally symmetrically arranged about aperture 23.

[0102] Also, as discussed hereinabove, since the centroid of themounting surface of the solder ball pad influences the position of thesolder ball, by employing the solder ball pad of the present invention,the solder ball pad layer 60 may be tailored to position solder balls asdesired or to correct for inaccuracy in the placement of apertures inthe solder mask 20. More specifically, in the case where solder maskplacement is less precise than solder ball pad layer 60 formation, thesolder ball pad layer 60 may be used to correct variances in the solderball mask aperture placement. Thus, each aperture in a solder mask 20could be measured against a desired placement, and then the solder ballpad layer 60 could be displaced in order to correct for the deviation.Correction may occur prior to formation of the solder ball pad layer 60;thus, aperture 23 position may be determined prior to forming the solderball pad layer 60 onto the substrate and the position of solder balllayer corrected accordingly. Alternatively, the solder ball pad layer 60of each mounting pad may be formed and then the solder ball pad layer 60may be modified to position a solder ball in a desired position. Forinstance, laser ablation, selective etching, or other removal processesmay be used to selectively modify the area of attachment of a solderball pad, and thus adjust placement of a solder ball attached thereto.

[0103]FIGS. 7G and 7H show embodiment 105 including a BPS solder ballpad configuration of the present invention where multiple apertures 77are formed in solder ball pad layer 60. Apertures 77 allow a solder ballto attach to the vertical sides thereof, thus increasing the surfacearea of attachment of a solder ball. Apertures 77 are shown as threecircumferential slots that are positioned over the surface of soldermask 20.

[0104]FIGS. 7I and 7J show embodiment 107 including a BPS solder ballpad configuration of the present invention where individual regions 93,95, 97, and 111 of solder ball pad layer 60 as well as the exposed area90 of the terminal pad 41 form the mounting pad 36. Therefore, anattached solder ball will be affixed to areas 80 and 90 as well as sidesurface 71 and the side surfaces of regions 93, 95, 97, and 111. Such aconfiguration may be advantageous to provide more surface area forsolder ball connection.

[0105] Many alternatives are possible, and the present invention is notlimited to any one configuration. Individual solder ball pad layer 60areas in combination with terminal pad 41 areas may form a mounting pad36. Although the present invention has been described herein asgenerally configured with a solder ball pad layer 60 that conforms tothe solder mask, thus creating a vertical depression consistent with theaperture 23 in the solder mask 20, the vertical surface of the solderball mounting pad 36 may be tailored as desired. For instance, it may beadvantageous to form the solder ball pad layer 60 so that it issubstantially planar on its top surface. Conversely, it may beadvantageous to create a vertical depression or tailor a verticaldepression so as to create increased surface area or to promote bondstrength or bond characteristics thereof.

[0106]FIGS. 7K and 7L show embodiment 109 including a BPS solder ballpad configuration of the present invention where solder ball pad layer60 forms areas 80 and includes aperture sections 113 that exposerespective areas 90 of terminal pad 41. Such a configuration may provideadditional surface area and vertical sidewall attachment of a solderball to areas 80 as well as provide an attachment area 80 to terminalpad 41. Thus, the mounting pad 36 of the present invention may compriseareas of solder ball pad layer 60 in combination with areas of terminalpad 41.

[0107] Referring now to FIGS. 8A and 8B of the drawings, a firstexemplary process flow for fabricating the inventive structure of thepresent invention as depicted in FIGS. 3A and 3B is illustrated. Asshown in FIG. 8A, a polymer conductive adhesive forming optionalinterface layer 38 may be applied over solder mask 20 and into aperture23. By way of example only, suitable conductive polymers in the form ofisotropic epoxy adhesives 3880 and 3889 are available from HenkelLoctite Corporation, Connecticut. The polymer interface layer 38 may beapplied by stencil printing to cover the surface of terminal pad 41exposed through aperture 23, the sidewalls of solder mask 20 definingaperture 23 and the top horizontal surface of solder mask 20 to form acollar of the polymer around aperture 23. An electroplated copper layer48 may then be formed over interface layer 38, followed byelectroplating of a nickel layer 18 a and a gold layer 18 b togethercomprising solderability enhancement layer 18, all as shown in FIG. 8B.It is also contemplated that a metal interface layer 38 may be appliedby stencil printing, followed by electroplating of the copper, nickeland gold layers.

[0108] Referring to FIG. 9 of the drawings, an optional interface layer38 to enhance adhesion to the solder mask 20 may be applied in the formof a nonconductive epoxy such as the aforementioned HYSOL®EO1073 andEO1075 compounds or other suitable polymer by stencil printing over thetop surface of solder mask 20 surrounding aperture 23, leaving theexposed area of terminal pad 41 and the sidewalls of solder mask 20defining aperture 23 free of material

[0109] In lieu of the use of a conductive or nonconductive polymer as aninterface process flow may proceed along several different paths.Referring to FIG. 10A, a “coverlay” element in the form of a dry filmphotoresist 100 (positive or negative) may be applied over solder mask20, patterned by exposure to a required wavelength of light through amask, developed, and portions of the dry film surrounding and extendinginto aperture 23 removed to expose terminal pad 41 and define annulus102 surrounding aperture 23. If a nonconductive polymer has been usedfor adhesion enhancement to solder mask 20, it may already be present onannulus 102. As shown in FIG. 10B a copper seed layer 104, which maycomprise a metal interface layer 38, may be electrolessly plated overthe dry film photoresist and into aperture 23, covering the exposedportion of terminal pad 41, which seed layer may be augmented byelectroplating if desired. Dry film photoresist 100 is then stripped offmechanically or chemically as known in the art, removing with it theoverlying copper and leaving the copper layer 48 within and surroundingaperture 23 contacting terminal pad 41 and extending from aperture 23 asa collar over the top surface of solder mask 20, as shown in FIG. 10C. Anickel layer 18 a and a gold layer 18 b may then be electroplated ontothe copper layer 48 to form solderability enhancement layer 18 andcomplete the structure of solder ball mounting pad 36 as shown in FIG.10D.

[0110] In another process sequence and referring to FIG. 11A, anoptional nonconductive polymer interface layer 38 (not shown) may beapplied to solder mask 20 in an area surrounding aperture 23. In eithercase, with or without the presence of optional nonconductive interfacelayer 38, an electroless copper layer 104, which may itself comprise aconductive interface layer 38, may be applied over solder mask 20, intoaperture 23 and over the exposed portion of terminal pad 41. A coverlayelement in the form of a dry film photoresist 100 (positive or negative)may be applied over solder mask 20, patterned by exposure to a requiredwavelength of light through a mask, developed, and portions of the dryfilm surrounding and extending into aperture 23 removed to exposeterminal pad 41 and define annulus 102 surrounding aperture 23 andexposing a portion of electroless copper layer 104, as shown in FIG.11B. Copper may then be electroplated onto the exposed copper in annulus102, over sidewalls of solder mask 20 defining aperture 23 and onto theexposed portion of terminal pad 41 to complete copper layer 48, as shownin FIG. 11C. Nickel layer 18 a and gold layer 18 b may then beelectroplated to form solderability enhancement layer 18, again as shownin FIG. 11C. Dry film photoresist 100 may then be stripped offmechanically or chemically, as known in the art, and the underlyingelectroless copper layer 104 removed by a soft etch comprising analkaline ammonia solution to complete the fabrication of solder ball pad36, as shown in FIG. 11D.

[0111] Although the foregoing description contains many specifics, theseshould not be construed as limiting the scope of the present invention,but merely as providing illustrations of some exemplary embodiments.Similarly, other embodiments of the invention may be devised which donot depart from the spirit or scope of the present invention. Featuresfrom different embodiments may be employed in combination. The scope ofthe invention is, therefore, indicated and limited only by the appendedclaims and their legal equivalents, rather than by the foregoingdescription. All additions, deletions, and modifications to theinvention, as disclosed herein, which fall within the meaning and scopeof the claims are to be embraced thereby.

What is claimed is:
 1. A substrate for an electronic device configuredfor mounting a discrete conductive element thereon, the substratecomprising: a sheet of insulative material; a metal layer defining aterminal pad formed on a surface of the sheet; an insulative maskextending over the sheet and having an aperture therein through which aportion of the terminal pad is exposed; and a bond pad layer comprisingat least another metal layer formed over, at most, a portion of theexposed portion of the terminal pad, the bond pad layer extending up asidewall of the aperture and over a portion of the insulative maskadjacent to the aperture.
 2. The substrate of claim 1, furthercomprising a solder ball in electrical contact with both the bond padlayer and the terminal pad.
 3. The substrate of claim 2, wherein thesolder ball is attached to a side surface of the bond pad layer.
 4. Thesubstrate of claim 3, wherein the solder ball is attached to the portionof the bond pad layer extending over the insulative mask.
 5. Thesubstrate of claim 2, wherein the bond pad layer is configured asradially extending elements generally symmetrically arranged about theterminal pad.
 6. The substrate of claim 2, wherein the bond pad layercomprises a plurality of apertures through which the terminal pad isexposed.
 7. A substrate for an electronic device configured for mountinga discrete conductive element thereon, the substrate comprising: a sheetof insulative material; a metal layer defining a terminal pad formed ona surface of the sheet; an insulative mask extending over the sheet andhaving an aperture therein through which a portion of the terminal padis exposed, the exposed portion of the terminal pad having a centroid;and a bond pad layer comprising at least a metal layer formed over atleast a portion of the exposed portion of the terminal pad, extending upa sidewall of the aperture and over a portion of the insulative maskadjacent to the aperture, and having a centroid; wherein the bond padlayer centroid is misaligned with the centroid of the terminal pad. 8.The substrate of claim 7, wherein the centroid of the bond pad layer ispositioned according to a measured lateral position of the aperture inthe insulative mask.
 9. The substrate of claim 7, wherein misalignmentbetween the centroid of the bond pad layer and the centroid of theterminal pad is attributable to removal of a portion of the bond padlayer.
 10. A method of forming a substrate for an electronic deviceconfigured for mounting a solder ball thereon, comprising: providing asheet of insulative material; forming a metal layer defining a terminalpad on a surface of the sheet; forming an insulative mask over the sheethaving an aperture therein exposing a portion of the terminal pad; andforming a bond pad layer comprising at least another metal layer whichis formed over, at most, a portion of the exposed portion of theterminal pad, the bond pad layer extending up a sidewall of the apertureand over a portion of the insulative mask adjacent to the aperture. 11.The method of claim 10, further comprising adjusting a centroid of thebond pad layer after it has been formed.
 12. The method of claim 11,wherein adjusting the centroid of the bond pad layer after it has beenformed comprises removing a portion of the bond pad layer.
 13. Themethod of claim 10, wherein forming the bond pad layer comprises formingthe bond pad layer with a lateral extent that exceeds the lateral extentof the terminal pad.
 14. The method of claim 10, further comprisingelectrically attaching a solder ball to both the bond pad layer and theterminal pad.
 15. The method of claim 14, wherein attaching a solderball to both the bond pad layer and the terminal pad comprises attachingthe solder ball to a side surface of the bond pad layer.
 16. The methodof claim 14, wherein attaching a solder ball to both the bond pad layerand the terminal pad comprises attaching the solder ball to the portionof the bond pad layer extending over the insulative mask.
 17. The methodof claim 10, wherein forming the bond pad layer comprises forming thebond pad layer as one or more radially extending elements generallysymmetrically arranged about the terminal pad.
 18. The method of claim10, wherein forming the bond pad layer comprises forming multipleapertures in the bond pad layer through which the terminal pad isexposed.